The present invention relates to the field of material science and, more particularly, to the field of material science which includes nanostructures. The present invention also relates to the field of chemistry and, more particularly, to the field of photocatalysis.
Material science has uncovered semiconductors with electronic properties that are strikingly beneficial to a number of desirable clean energy and environmental technologies based on solar-driven photocatalysis. Of a range of materials that have been investigated in pursuit of greater utilization of solar energy, currently the most effective semiconductor for photocatalysis is titanium dioxide (TiO2), which absorbs light from only the ultraviolet (UV) portion of the solar spectrum. That is, TiO2 has a band gap that corresponds to the energy of a UV photon. As a consequence, TiO2 can absorb but a small fraction of solar radiation, leaving over 90% of the energy in the solar spectrum essentially wasted. Narrowing the band gap of TiO2 is therefore vital to achieve efficient absorption of sunlight, which is true for all wide band gap semiconductors if they are to be used in an energy conversion process driven by solar radiation. While impurity doping is a well-established method of tuning the band gap, its application to TiO2 has had only limited success.
Solar radiation is an energy resource that can be used to produce electricity and clean fuel, or used in combination with selected semiconductors to induce environmentally important photocatalytic reactions such as air purification and water decontamination. Effectiveness of solar-driven photocatalysis is determined to a great extent by the semiconductor's capability of absorbing visible and infrared light, in addition to the requirement of a large surface area that can facilitate a fast rate of surface reactions. Nanostructured TiO2 has emerged as a unique wide band gap semiconductor photocatalyst that plays a key role in a variety of solar-driven clean energy and environmental technologies (see, e.g., Grätzel, Photoelectrochemical cells, Nature, 414, 338-344 (2001); Hoffmann et al., Environmental applications of semiconductor photocatalysis, Chem. Rev., 95, 69-96 (1995); and Fujishima et al., TiO2 photocatalysts and related surface phenomena, Surf. Sci. Rpts, 63, 515-582 (2008)). Nevertheless, despite decades of extensive research, the true potential of TiO2 has not been realized, as the material absorbs only in the UV portion of the solar spectrum.
To overcome limited absorption of solar radiation by TiO2, extensive efforts have been made to vary its chemical composition by adding controlled metal or non-metal impurities that generate discrete donor or acceptor energy states in the band gap (see., e.g., Asahi et al., Visible-light photocatalysis in nitrogen-doped titanium oxides, Science, 293, 269-271 (2001)). Through such impurity doping, the solar absorption characteristics of TiO2 have been improved to some extent. For example, when non-metal light-element dopants are introduced, absorption by TiO2 can be modified as the result of electronic transitions from the dopant 2p or 3p orbitals to the titanium 3d orbitals. Nitrogen-doped TiO2 so far exhibits the best response to solar radiation, but its absorption in the visible and infrared wavelength portions of the solar spectrum remains inefficient. For example, see Chen et al., The electronic origin of the visible-light absorption properties of C-, N- and S-doped TiO2 nanomaterials, J. Am. Chem. Soc., 130, 5018-5019 (2008), which reported that N-doped TiO2 has: (a) a band gap of 3.0 eV; (b) an absorption spectrum that exhibits a decreasing absorbance from 415-550 nm (a shoulder) and a diminishing absorbance from 550-800 nm (a tail); and (c) no absorption above 800 nm (i.e. an absorption edge of 800 nm).